1. Field of the Invention
The present invention relates to a data processing apparatus for processing code data or image data.
2. Related Background Art
Various printing apparatus, such as laser beam printers, have rapidly become popular in recent years.
Such printing apparatus can be divided into a controller unit and a printer unit in view of their functions. The controller unit receives various data such as character code data from a host computer, for example, edits said data as image information (dot information) in units of a character or of a page, and sends said information as an image signal together with various print control signals to the printer unit. Said printer unit is equipped with a sheet feeding mechanism and an image forming mechanism having a semiconductor laser unit, a photosensitive drum etc., and effects a printing operation on a recording sheet, according to the control signals and image signals from the controller unit. In the following description, the interface signals between the controller unit and the printer unit, including the image signals and the control signals such as commands or status signals, will be collectively called a video interface signal.
However, in such printing apparatus, well improved efficiency of use of the printer unit is not always obtained because only one controller unit can be connected to the printer unit. Also such printer is uneconomical because each host apparatus has to be equipped with a printer unit.
The above-explained printing apparatus are disclosed for example in U.S. Pat. No. 4,823,192, 4,786,923, 4,866,555 and 4,835,618 and in U.S. Pat. application Ser. No. 07/554,187, filed Jul. 20, 1990 (effective U.S. Filing data Apr. 23, 1987), but further improvements have been desired.